The regulations against the emission of NOx caused by combustion are intensified year after year, and very active technical challenges to decrease NOx emissions are being conducted. NOx generated by combustion includes fuel NOx, prompt NOx and thermal NOx. Among these types of NOx, thermal NOx is produced as the nitrogen molecules in combustion air are oxidized in a high temperature atmosphere, and is highly dependent on the temperature. At higher combustion temperatures, NOx production increases sharply. Thermal NOx is produced without fail if the combustion gas contains nitrogen molecules, and especially when a hydrocarbon-based fuel is burned, the NOx emitted is said to be mostly thermal NOx. A number of methods for decreasing NOx are proposed, including multi-stage combustion methods, exhaust gas recirculation methods, and lean combustion methods. It is also proposed to combine these methods in many ways.
In multi-stage combustion methods, the fuel or combustion air is divided for combustion in two or more stages, and it is intended to achieve low NOx combustion by keeping the flame temperature low or keeping the oxygen concentration low. These combustion methods have a problem in that the multi-stage combustion makes the burner complicated. The exhaust gas recirculation methods are intended to lower the flame temperature or lower the oxygen concentration by mixing part of the combustion gas with air or fuel, and includes forced exhaust gas recirculation methods and self-induced exhaust gas recirculation methods. The forced exhaust gas recirculation methods use a recirculation duct and blower to forcibly mix parts of the combustion gas with combustion air or fuel. These are the most general methods. In the self-induced exhaust gas recirculation methods, a specially devised burner is used to have combustion air flow or fuel flow encapture the combustion gas for mixing to achieve the effect of exhaust gas recirculation by the jet entrainment on encapturement. The self-induced exhaust gas recirculation methods have an advantage in that the effect of exhaust gas recirculation can be obtained without forcibly recirculating the combustion gas, and is free from the complication of the multi-stage combustion methods in that the fuel or combustion air is divided into a plurality of lines. A burner adopting a self-induced exhaust gas recirculation method is disclosed, for example, in Japanese Laid-Open No. 87-17506, and many other burners use the self-induced exhaust gas recirculation methods. However, these methods are limited in their capability to decrease NOx and, further, technical development is necessary to meet the latest severe NOx regulations. Combustion methods developed to maximize the advantage of self-induced exhaust gas recirculation are proposed in Japanese Patent Laid-Open No. 89-300103 and 91-91601, and Japanese Utility Model Laid-Open No. 77-61545. These combustion methods are characterized in that combustion air flow and fuel flow are separately and independently injected into a furnace with a burner without any flame stabilizing mechanism to maximize the effect of self-induced exhaust gas recirculation. In this configuration, the flame is not stabilized in the burner, but is formed at a lifted position, and the combustion begins after part of the combustion gas in the furnace has been sufficiently entrained or encaptured by the fuel flow or combustion air flow. In these combustion methods, the flame is a diffusion flame. Since there is no flame stabilizing mechanism, it can happen that unless the temperature is high, stable ignition cannot be achieved. Therefore, even though the methods are suitable for high temperature furnaces, such as heating furnaces and melting furnaces, they have problems in that the amount of unburned fuel increases and a larger furnace must be used for perfect combustion when they are applied to boilers and low temperature heating furnaces.
Another method for reducing thermal NOx is to use a premixed flame. Premixed combustion at a high increase air ratio can significantly decrease NOx, but a high excess air ratio is likely to decrease the efficiency of combustion and heat transfer. Furthermore, the premixed flame is poor which is disadvantageous.
A method of decreasing thermal NOx by combining the premixed combustion with the effect of self-inducted exhaust gas recirculation was proposed in Japanese Laid-Open No. 91-175211. In this combustion method, the flame stabilizer is a special device, and part of the low temperature combustion gas is mixed with the premixture before the premixture initiates combustion to lower the flame temperature, or to lower the oxygen concentration for decreasing NOx. This combustion method and apparatus presented problems which were observed with other premixed type burners, such as an air-fuel mixer is necessary to generate a premixture for premixed combustion, and since a premixture within inflammable limits is used, the flame may go back into the burner or mixer.
Furthermore, there is a problem in that since part of the combustion gas is mixed with an inflammable premixture, which could result in ignition occurring immediately after the mixing between the premixture and the combustion gas, if the mixed combustion gas is high in temperature, the effect of self-induced exhaust gas recirculation cannot be sufficiently used. Therefore, the flame stabilizer must be specially devised to ensure that the premixture is not ignited when the premixture and part of the combustion gas are mixed.
As described above, self-induced exhaust gas recirculation methods have advantages in that the burner can be simple and low NOx combustion is possible, compared with other low NOx combustion methods, such as multi-stage combustion methods and lean premixed combustion methods. In the combustion methods for decreasing thermal NOx by using self-induced exhaust gas recirculation, if the self-induced exhaust gas recirculation is used to the maximum extent for the diffusion flame, the unusable temperature range useable in the furnace is limited, and the useable combustion equipment is also limited, which is disadvantageous. Moreover, the application of self-induced exhaust gas recirculation to the premixed flame has the problem of flame stability peculiar to the premixed combustion, like back combustion, and requires a more specifically devised flame stabilizer, which is disadvantageous.